"Aftermarket" is a term used to describe parts that are not made by the factory. So for example, for your Chevrolet Monza, if you buy parts for your car that are not actually made by Chevrolet, then those are termed "aftermarket". Many people use the term "Monza aftermarket parts" specifically when referring to custom parts for the Monza, like performance parts or custom appearance parts. But even if you just replace your Monza brake pads with OEM-grade factory-style brake pads that are not made by Chevrolet, those too are aftermarket Monza parts. Over the course of a vehicle's life, it almost certainly will have aftermarket parts installed in it at some point, even if the vehicle is never actually "modded out".